A deep rumbling sound was the first sign of trouble, but when train cars plowed to a sudden, jumbled halt — poking out at disturbing angles from the industrial buildings that hug the rail line — it was clear another train had crashed on this busy, straight stretch of track.

At 3:28 p.m., Sunday, the first call was made to 911 from a resident of Wickens Avenue, where the backyards of bungalows look out across the tracks, and he was quite specific: A train had derailed.

It took two minutes for the stunned and injured passengers and crew aboard the six cars of VIA Rail Commuter Train 92 to put finger to phone; the first 911 call from inside the damaged cars came at 3:30 p.m. Within minutes, seven calls from inside alerted emergency crews to the work ahead: They spoke of chaos, blood, injuries, twisted metal and broken glass.

One caller asked for an ambulance.

First emergency crews and then transportation investigators rushed to the Burlington site of the railway linking Niagara to Toronto.

By Sunday evening, the human toll was known: dead were two locomotive engineers and a trainee, and seriously injured were three passengers; 42 other passengers were taken to hospital, where eight remained on Monday.

Faisal Abid, co-founder of software development firm Matchfuel, was in seat nine of the passenger car immediately behind the locomotive. “There were people falling on top of me, people falling behind me, stuff was hitting me,” he said, describing the final moments as the car fell on its side and ground to a halt on the trackside gravel.

Until they could be pulled out by paramedics and firefighters, passengers found themselves trapped in the sideways car, now turned into a chaotic mess of luggage and bent seats. “There was a lot of yelling and a lot of blood, a little girl was crying because her hand was bleeding — it was chaos.”

A man behind Mr. Abid was thrown clear through the window of an emergency exit, coming to rest back first on the ground. “His wife was saying he already had a bad back so he was in a lot of pain,” said Mr. Abid.

By Monday, the investigation shifted to the mechanical.

The train derailed at the very moment it was switching tracks, when trains on this stretch must slow from 80 miles per hour to 15 miles per hour. (Rail lines, by international convention, do not use the metric system.)

Whether Train 92 reduced speed as it should is not yet known. The answer lies in the so-called black box — the event recorder, which, in fact, is a bright orange metal case — taken by the Transportation Safety Board of Canada (TSB), where it is being examined.

It might also help reveal who was behind the controls at the time, and whether it was the trainee.

VIA engineer-in-training Patrick Robinson, 40, of Cornwall, Ont., was a new VIA employee on board as an observer. He joined VIA in October 2011. He had a young daughter and son.

Also killed were Ken Simmonds, 56, and Peter Snarr, 52, both from Toronto.

Mr. Simmonds leaves behind a wife and one daughter. He joined VIA as a locomotive engineer in 2007 and started his career with CN in 1979. Mr. Snarr leaves behind a wife and two daughters. He also joined VIA as a locomotive engineer in 2007. He started his career with CN in 1978.

“There were three crew members in the head end of the train. As to who was on the controls, right now we’re still investigating that,” said Tom Griffith, TSB regional senior investigator.

The black box will tell investigators the speed and what controls were being activated, such as the train’s whistle or brakes, at the time of impact. It does not record audio or video, as is often the case for airplanes.

After the train derailed, it slid into a building, destroying the locomotive cab where the three employees were.

“They didn’t stand a chance,” Mr. Griffith said.

If Mr. Robinson was at the controls, he would have been working under strict supervision, he said.

While Mr. Robinson was new to VIA, he was an experienced engineer, working mostly on freight trains, said VIA spokesman Malcolm Andrews. “He worked for two or three different railways, including CN, since 1990, in various positions, including locomotive engineer.”

Part of the VIA training includes a familiarization process where new engineers ride with experienced engineers on routes where they might eventually work, Mr. Andrews said.

“We are going through the coaches to see why the injuries occurred; if there is something in there that, other than the speed of the train, caused the injuries, if seats came loose or whatever,” said Mr. Griffith.

The TSB obtained an initial download of information from the recorder, but investigators “will have to go a little farther” to get the rest of the information because of damage, he said. The train cars were slowly erected and taken to an isolated area for further investigation.

Police are conducting a separate investigation.

“Our primary role is to determine if anything criminal has taken place here,” said Sergeant Dave Cross of Halton Regional Police, including vandalism or interference.

Late Monday, Halton police were still trying to locate about a dozen of the 75 passengers and appealed for anyone who has not spoken with an officer to call.

Police have a passenger manifest but not everyone on it has been accounted for, likely having left the scene, uninjured, by private transportation, said Sergeant Paul Davies of the Collision Reconstruction Unit.

Worker representatives moved to dispel concerns about the three employees’ abilities. “I can tell you that the guys involved were senior employees, well-experienced employees,” said Teamsters Canada Rail Conference President Rex Beatty.

“I would suspect that they were performing their duties as they normally would and that this all caught them by surprise… My understanding is that they were dedicated employees doing their job and something went horribly wrong,” he said.

The importance of the probes was made clear when it was announced that a class-action lawsuit was already being prepared.

Sutts, Strosberg LLP will likely press a multimillion-dollar class-action, said Sharon Strosberg. The firm successfully led a suit against VIA and CN after a 1999 crash.

This derailment happened on a straight track in good weather, near the site of a 2008 freight train derailment. On Feb. 18, 2008, a broken wheel on one of CN’s rail cars caused 19 freight cars to jump the tracks, causing delays and cancellations, but no injuries.

As investigators probe, people mourn.

A middle-aged woman whose daughter works for VIA arrived near the crash site. She taped three red roses to a street sign; attached was a note: “God bless your souls my heart goes out to the families of the fallen ones. May God hold you in his arms and console you in your hour of need,” it read.